Probe-based instruments have been developed that obtain information via interaction between its sensing element and a sample being analyzed. One such instrument that is typically used for nano-scale and even atomic-scale analysis is a type of scanning probe microscope (SPM) referred to as an atomic force microscope (AFM). It is very versatile because it can analyze conductive and non-conductive samples since the sample may not be electrically charged during operation.
The sensing element of an AFM is a probe that interacts with the sample during operation to obtain information about the sample, such as size, surface contour or topography, shape, roughness, atomic makeup, molecular makeup, and/or other characteristics. The cantilever often includes a probe tip, e.g., stylus, and is scanned over the sample by moving it and/or the sample relative to one another in a manner preferably in a raster scan pattern. The analysis produces data from tip-sample interaction at numerous locations of the scan, which often is used, for example, to generate a topographic image depicting the outer surface of the sample that was scanned. More generally an SPM measures any number of properties of a sample, including for example surface topography, magnetic forces, electric forces, temperature, thermal conductivity, electrical properties, friction, elasticity, adhesion, just to name a few. These and other data representative of sample properties are typically measured by using a probe and detection system that can convert a probe sample interaction into data that is indicative of the property of interest. For example, in the case of magnetic measurements, an AFM tip is coated with a magnetic material and the resulting force between a sample region and the AFM tip is detected.
The cantilever and tip (where so equipped) form a probe that is received in a mount, e.g., probe mount, of the AFM. An AFM probe has at least one cantilever that extends outwardly from a support typically referred to as a base, substrate or chip and that can be part of the mount. The cantilever typically is elongate, narrow and relatively small to achieve nano-scale and atomic-scale imaging. For most such applications, the cantilever usually is no longer than about 500 microns and no wider than about 50 microns, and typically much smaller. The tip is also quite small and usually quite sharp, typically having a radius or diameter between three and fifty nanometers.
During operation, the cantilever is scanned over the sample, typically in a raster pattern, to analyze the sample. As the cantilever moves along the sample surface, its tip can move up, such as when a bump on the sample is encountered, can move down, such as where there is a depression or sidewall in the sample, and, in certain instances, can move side-to-side, such as when friction is encountered or when CD imaging of semiconductor sidewalls is performed.
Relative movement between the sample and cantilever is controlled to help position the tip so it either contacts the sample or is close enough to the sample that interaction between the cantilever tip and sample occurs. This interaction can be caused by friction, such as tip-sample friction, molecular force, including strong and weak, e.g., van der Waals, molecular forces, as well as magnetic force, for example. This interaction is desired because it is measured during operation to analyze the sample, including when imaging the sample.
Controlling relative movement can be done in many ways but typically is achieved using one or more selectively controllable actuators. These actuators, typically of piezoelectric construction, are used to either move the cantilever relative to the sample, the sample relative to cantilever, and, in some instances, both of them at the same time. For example, in one well know arrangement, at least one actuator is used to move the cantilever, and hence its tip, toward or away from the sample along the Z-axis. One or more actuators are typically used to provide relative movement between the cantilever and sample along either or both the X-axis and the Y-axis. One well known positioning device used in AFMs employs one or more actuator that provides motion in up to three orthogonal dimensions, and is referred to as a scanner. Suitable examples of such high-resolution, three axis scanners are disclosed in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980. The scanner may be a single unit that provides motion in three directions or may be made from discrete units. As mentioned above, the relative scan motion can be created by scanning the probe, scanning the sample, or any combination thereof. For example, some AFMs are constructed to scan the sample in the XY direction and actuate the probe in the Z-direction.
As the sample is scanned during AFM operation, the cantilever deflects in response to interaction with the sample. A detector is used to measure this interaction, typically by measuring cantilever deflection. This is because changes in interaction force, such as due to a change in sample surface height or sample surface roughness, cause cantilever deflection to change in a corresponding, even proportional, manner. Therefore, measuring the changes in cantilever deflection that occur during scanning provide information from which various sample characteristics, including the contour of its out surface, can be obtained.
One commonly used detector uses the cantilever as an optical lever that reflects a beam impinging against it during scanning. Any change in cantilever deflection causes a corresponding change in the incident angle of the reflected beam, which is measured by measuring reflected beam movement using an optical sensor on which the reflected beam shines. One suitable and preferred optical sensor uses one or more photo-detector sensors or the like arranged in an array, such as in a side-by-side or even quadrature array, to sense any changes in reflected beam position. During scanning, output from the array is collected and processed so as to enable an image of the sample surface scanned by the cantilever to be produced in a manner where it can be displayed, printed, further analyzed, etc. An example of one such detector suitable for AFM use is disclosed in Hansma et al. U.S. Pat. No. RE 34,489, which uses a laser to produce the beam for the optical lever arrangement.
Since AFMs are often capable of operating in more than one mode, the nature of interaction between the cantilever and sample is usually, if not always, mode dependent. For example, a cantilever of an AFM operating in contact mode will encounter a different type and magnitude of interaction than when operating in a non-contact mode, e.g., oscillating mode. Where contact is direct, interaction obviously tends to be direct as forces are directly transmitted between the cantilever and sample. Where interaction is less direct, such as when it is indirect, e.g., non-contact mode, the nature of cantilever-sample interaction can prove relatively complex. For example, it is not unusual for interaction to be due to one or more of weak molecular force attraction, such due to Van der waals forces, stronger atomic-based forces, repulsive forces, and adhesive forces due to contact with a hydrophilic layer on top of the sample, which also can be dependent on the makeup of the sample as well as how close the cantilever tip is to the sample.
In contact mode operation, the probe is typically scanned across the surface of the sample while keeping the force of the tip of the probe against the surface of the sample generally constant. This is usually accomplished by moving either the sample or the probe vertically relative to the surface of the sample being analyzed in response to sensed deflection of the cantilever as the probe is scanned generally continuously across the surface of the sample. Vertical motion feedback information can be stored, along with other tip-sample interaction feedback information, and used in constructing an image or the like representative of the surface of the sample that corresponds to the sample characteristic being measured, e.g., surface topography.
Depending on the AFM, some AFMs can operate in different types of oscillation modes including where the oscillation frequency is tied to a resonant frequency of the cantilever. Feedback normally is used to keep a parameter of cantilever oscillation (e.g., amplitude, phase, frequency) constant. The information providing the feedback typically is at least partly based on cantilever-sample interaction, e.g., tip-sample interaction, taking place during operation. As is preferably also the case with contact mode operation, information obtained using cantilever-sample feedback is collectable, storable, and usable as data to analyze the sample, including by characterizing it as well as imaging it.
One particularly advantageous and versatile oscillating mode is TappingMode™ AFM (TappingMode™ is a trademark owned by Veeco Instruments Inc. of Santa Barbara, Calif.), which is implemented by oscillating the cantilever at a frequency at or near its resonant frequency. Cantilever oscillation amplitude, phase or frequency preferably is kept substantially constant via feedback, information for which is obtained from the cantilever-sample interaction, e.g., tip-sample interaction, that takes place during operation. In intermittent contact or TappingMode AFM the tip only periodically contacts the sample surface generally according to the drive signal, thus making it a lower force mode operation than a mode such as contact mode where the tip and sample are substantially continuously engaged.
No matter which operational mode is chosen, setup and operation of a measurement instrument as complex and versatile as an AFM can be time consuming and tricky, especially for a novice AFM operator. For example, setup and operating parameter values typically depend on factors such as the type of sample material including whether it is hard or soft, conductive or non-conductive, organic, synthetic or biological in nature, among other things.
While past attempts have been made with AFMs to automatically adjust gain to minimize the difference between trace and retrace data, this method also has not proven particularly effective. For example, it may not be able to handle sample topography and operating parameters, such as, setpoint, actuator hysteresis and tip shape, can unpredictably and adversely impact trace-retrace differential data such that any attempt to control it through gain adjustment is largely ineffective.
This is not surprising in view of the numerous scan parameters that must be taken into account in AFM setup and operation along with those that can require adjustment during operation. For example, a user may need to adjust such scan control parameters as setpoint, scan speed, proportional gain, integral gain, drive frequency, drive amplitude and other parameters. Without great care, considerable experience, and sometimes a little luck, tip, cantilever or sample damage can occur, poor or unusable results can be obtained, and, in instances where everything appears to go well, operation inefficiencies can be so great that scanning time is nowhere near optimal thereby wasting a considerable amount of time, which is particularly problematic for high throughput applications such as those of the semiconductor industry.
For example, at present, if the value any one of several control parameters manually selected is not at or within a reasonable range of its optimum, poor performance and unacceptable data will very likely result. In addition, relatively complex interdependencies existing between certain AFM parameters often make setup a trial and error procedure, even for the most experienced AFM operators.
In performing AFM setup, the values for several control parameters must be set along with feedback loop gains for different operational modes and other instances where setting up such gains is required. Setup must take into account and configure for parameters such as scan size, pixels per line, number of scan lines, scan rate, tip scanning speed, digital-to-analog (D/A) resolution, Z-center position, i.e., Z-center voltage or the center of the Z piezo operation range, tip wear control, and sample damage minimization. When an AFM is set-up to operate in oscillatory mode, such as TappingMode™, setup must include choosing an amplitude and setpoint associated with the oscillation.
When an AFM is going to operate in an oscillatory mode, such as TappingMode™, initial values for integral gain, i.e., I-gain, and proportional gain, i.e., P-gain, may also be manually set. Selecting gain values can be tricky because it typically depends on factors such as the nature of the oscillatory mode being employed, sample topography, the type of sample and medium in which it is located, as well as other factors. For example, where gain is set too low, system response tends to be relatively slow, which can result in the tip not following the sample surface. Where set too high, the feedback loop can start oscillating or backfeeding upon itself, which can undesirably add considerable noise to the sample image being generated.
In addition, the initial gain setup may be fine initially, only to be unsuitable later one some other factor, such as topography changes. For instance, where the sample is relatively rough, gain typically should be set higher in order to image such high featured topography with any resulting increase in feedback oscillation noise being tolerable. Where the sample is relatively smooth or flat, gain should be set lower to minimize noise. By keeping noise low by keeping gain low, better resolution of flat areas is achieved thereby enabling the AFM to better image its finer details. However, as understood in the field, excessive noise can adversely affect imaging along flatter areas of the sample where an initially high gain setting ends up being too high when the sample flattens out. Conversely, an initial low gain setting frequently impedes imaging of higher features of the sample producing an image with such higher features being either distorted or missing.
These setup considerations become even more problematic when operating in an oscillating mode, such as TappingMode™. For example, since the highest useable gains when operating in TappingMode™ typically depend on cantilever dynamics, setting gains becomes further complicated because cantilever dynamics, in turn, is a function of the free air tapping setpoint. Indeed, factors such as cantilever dynamics and Z-actuator response speed can create such difficultly in setting the initial setpoint and gains, the operator often resorts to trial and error until the sample image starts to look good.
Unfortunately, because one can affect the other, trial and error can go on for a long time. For example, as setpoint is lowered, gain can be set higher and vice versa. However, while lower gains permit a lower setpoint to be used, which typically increases cantilever response, it also increases error generation rate, which can undesirably blur or otherwise distort the image being produced during scanning.
In the end, what often results is the operator setting some initial parameter values, gains and setpoint and then manually adjusting the value of each, one-by-one until feedback oscillation occurs and then backs off. While this process may work reasonably well for experienced AFM operators, it is inefficient, time consuming, and quite often, less than optimal. In addition, it does nothing to address the dynamic nature of AFM imaging, which often requires an operator to either change certain settings on the fly during operation or to observe the image, etc., and go back an re-scan those parts of the sample that are poorly imaged with adjusted parameter values. Once again, extremely slow, inefficient and non-optimal.
Expert user intervention is typically also required during pre-scan or image acquisition processes to maintain probe-sample interaction according to the mode of operation. The above-described scanning probe microscopes (SPM) employ a uniaxial feedback control system to maintain probe-sample interaction while scanning. That is, such a system is capable of actuating to alter probe-sample distance along the Z-axis, and also of sensing probe response to sample surface position along the Z-axis. Moreover, SPM engage establishes initial probe-sample interaction, whether continuous or intermittent. Successful maintenance of a continuous (contact mode) or intermittent (TappingMode™) target interaction while scanning in a direction other than Z demonstrates good tracking of the sample surface by the probe.
Such tracking fails when the probe ceases to interact with the sample. This is more apt to happen under any of the following conditions including, but not limited to, (1) in the scan direction, the sample surface slopes away from the probe; (2) the SPM is scanning rapidly; (3) the feedback loop gain is set low; and (4) at the chose setpoint, probe-sample interaction is weak. In one known tracking measurement technique, a trace-minus-retrace (TMR) operation may be performed. Wherein the same N (e.g., 512) consecutive data acquisitions on the sample surface measured scanning in one direction, are individually subtracted from samples taken from obstensibly the same sites in reverse order during retrace of the same sample surface line. Problems have been demonstrated with this technique and thus an improvement has been desired in the AFM field.
Hence the need has arisen for a method and arrangement that is able to automate AFM setup, and do so to achieve optimal tracking between the tip and sample. What is further needed is the same or a like method capable of being used during actual AFM scanning.